Histone gene expression is tightly coupled to DNA replication, providing a viable opportunity to directly investigate regulatory mechanisms that are operative during proliferation of hematopoietic stem cells. The goal of this project is to experimentally address transcriptional regulation of histone genes in hematopoietic progenitor cells as a model to define factors that control cell cycle progression. Results obtained can be used to facilitate expansion of cell populations and competency for engraftment and differentiation. Previous studies of promoter sequences and transcription factors that influence cell cycle regulation of histone gene transcription have shown that Site II, the principal cell cycle regulatory element of the histone gene, interacts with cyclin A, CDC-2, and RB-related proteins, as well as IRF-2. These key regulatory factors control both cell growth (tumor suppression) and cell cycle regulatory phosphorylation events. Thus our working hypothesis is that selective occupancy of the proximal regulatory elements of the cell cycle dependent histone gene promoter is linked to growth control in hematopoietic stem cells and to the downregulation of cell growth that accompanies and potentially serves as a determinant for expression of post-proliferative phenotypic properties. We will focus on modifications of the factors required for cell cycle stage- specific transcription of the histone gene and, equally important, for the downregulation of transcription postproliferatively at the onset of differentiation. Our specific aims are to; 1) characterize the complement of transcription factors that interact with the principal histone gene promoter regulatory elements under cell cycle and cell growth conditions functionally relevant to hematopoietic progenitor cell activity; 2) assess effects of transcription factors modulation on control of histone gene expression in hematopoietic stem cells; 3) address phosphorylation- mediated signalling mechanisms that control activities of histone gene regulatory factors; and 4) determine the consequential effects of modulating cell cycle dependent transcription factor expression in hematopoietic cells on engraftment in myeloablated mice as well as subsequent growth and differentiation.